1. Field
The present invention relates generally to lithographic formation of integrated circuit patterns, and more particularly to a method for generating the spatial frequency modulation of a lithographic pattern by projecting a light beam that has been modulated with a set of spatial frequencies through a hologram modulated by a signal having a second spatial frequency modulation.
2. Description
The resolution of a lithographic image is limited by the wavelength of the light that forms it. Currently, source wavelengths shorter than λ=193 nm (e.g., as provided by an ArF excimer light source) are not contemplated for IC manufacture until the future era of soft x-ray lithography. Fortunately, wavelength is reduced inside a medium, and a favorable reduction of as much as 1.8× can potentially be obtained for propagating waves within photoresist films. This corresponds to an upper limit of 1.8 for the resist refractive index for example. Current resists typically have refractive indices of around 1.5 to 1.7 when sensitized.
Considerably larger reductions in the effective wavelength are possible in images formed from evanescent waves. However, evanescent waves can only exist in extremely thin films, and even the somewhat larger thicknesses that are typically given to photoresist films (of order 0.1-1.0 microns) are microscopic on the scale of the lens elements which propagate the light from mask to wafer. With the possible exception of imprint templates for specialized applications, practical semiconductor manufacturing requires that circuit patterns first be fabricated on 4× expanded masks, and then de-magnified to final size using a projection lens. The macroscopic size of conventional projection lenses makes them unsuitable for exploiting waves that become evanescent in any medium between the resist film stack and the mask.
In fact, until recently, lithographic projection lenses were incapable of introducing spatial frequencies into the image that corresponded to wavelengths shorter than the vacuum wavelength. This is because the waves had to propagate through an air space between the last lens element and the wafer film stack (with air having essentially the same n=1 index as vacuum). A solution has recently been developed to overcome this limitation, namely filling the intervening space with water, whose index is about 1.44 at 193 nm. Note that a spatial frequency component at the 1.44 limit would propagate parallel to the wafer surface in such water immersion systems, with no projected component of propagation along the lens-to-wafer axis. In practice image spatial frequencies are therefore gated by a smaller effective index of about 1.35, since diffraction-limited transfer from a lens of reasonable diameter to an image of reasonable field size entails an upper limit on the propagation angle of order 70°. Because of this optical design margin, the maximum NA in lithography lenses is limited to roughly 0.93 times the refractive index of the coupling medium.
Increasing the refractive index of the immersion fluid is challenging with a 193 nm source wavelength, because high index fluids tend to have high absorption in the deep UV. Viscosity, defects, and evaporatively or absorptively driven thermal fluctuations also become concerns, as does chemical stability and compatibility under UV loading. It is not clear whether these problems can be solved with any 193 nm immersion fluid of index higher than water. Moreover, the lens elements in 193 nm systems also face stringent materials requirements, and their refractive index is currently limited to n=1.56 or below. The optical design margins noted above show that lens element refractive indices would gate the NA to a maximum of about 1.45 even if an acceptable high index immersion liquid were possible. This is because the final lens element generally cannot have a concave exit surface since the immersion cavity would then become impractically thick along the optical axis, greatly stiffening the already difficult-to-surmount materials requirements that a hypothetical high-index immersion fluid must meet.
In fact, it is currently believed that the materials challenges involved in significantly raising the refractive index of the final lens element are at least as difficult as those faced in raising the index of the coupling medium. Achieving the necessary freedom from minute levels of index in-homogeneity and uncorrectable birefringence is extremely difficult at 193 nm, given the element sizes and tolerance levels needed for advanced lithography lenses. Thus, with existing methods of projection lithography the spatial frequency content of the projected image ends up being gated by an effective wavelength of about λ/1.35, potentially limiting the miniaturization of future semiconductor designs. There are a few known approaches for addressing this problem. First, the spatial frequency limit can be improved by numerical factors through techniques like multiple patterning, or frequency doubling (e.g. by using phase shift masks or oblique illumination).
The first class of methods, so-called multiple patterning methods, are theoretically capable of solving the index-limited resolution problem of conventional single exposure/patterning methods. (Multiple patterning methods involve using more than one mask exposure and pattern transfer to print a single level of the integrated circuit, instead of the single exposure and patterning that are traditionally used.) Multiple patterning can avoid the spatial frequency limit because the frequency content of a printed chip level can theoretically be stepped down arbitrarily by carrying out the pattern transfer of successive closely-neighboring features in separate steps, with each separate image including only one of the features at a time. However, such processes require that the patterns transferred in any additional exposure be very narrow in order to allow subsequently transferred features to fit in the gaps between them. Unfortunately, even with one dimensional (1D) layouts, the tight transfer process control needed to print a narrow feature from a wide image will usually prevent a doubling of minimum manufacturable resolution from being feasible by this means—resolution improvements are typically quite a bit less than 2×. Even under ideal conditions, achievement of more than a doubling of resolution would require three or more transfer steps, which would cause cost to rise very substantially. Note that with two-dimensional (2D) patterns, a full doubling of resolution in every cross-section (e.g. along both the x and y axes) is not even theoretically possible if only two exposures are used.
Current lithographic practice exploits frequency doubling to improve resolution. Frequency doubling is based on the fact that photoresist responds in an identical manner to the positive and negative half-cycles of each amplitude harmonic. Since exposing intensity is the square of the electric field, the spatial frequency bandlimit in the exposing intensity image is twice that of the incoming amplitude spatial frequencies. This means, for example, that one can print a periodic array of patterns with twice as fine a periodicity by alternately tiling each repeat with positive or negative phase. It is possible to take advantage of this enhancement in resolution without insisting that the imaged pattern take the form of positive and negative replicas in exact balance; however when such a balance is present the patterns have no amplitude in the DC order, also known as the zero order, and classic frequency doubling obtains.
Frequency doubling can be implemented (when patterns are compatible) by appropriately applying opposite phase shifts to neighboring features on the reduction mask; alternatively, a broadly equivalent effect can be achieved by illuminating the mask from an oblique set of directions. These techniques have become widely used, and have significantly improved the resolution capabilities of semiconductor technology. However, this classic approach is inherently unable to provide more than a single doubling of resolution. New methods will be needed further extend the spatial frequency content of lithographic images. Such methods should preferably be compatible with the current practice of exploiting frequency doubling.
One antecedent projection lithographic technology in the prior art include: Thin Film Interference Talbot Lithography (also known as In-Situ Interference Lithography). In Thin-film interference, e.g., Talbot lithography, employs a so-called “parent” grating (of lines and spaces) that is fabricated on an initial layer of, for example, photoresist as shown in FIG. 1. Interference lithography is a well-known method for fabricating such a grating. As shown in FIG. 1, when illuminated from above by a normally incident plane wave, it is known that (in the conventional Fresnel diffraction regime) such a grating 10 will produce duplicate images of itself in periodically spaced planes below the grating lines, and further that denser images having a two-times finer pitch will be produced in intermediate planes that lie midway between the planes of the duplicate images. Such three-dimensional (3D) interference structures are known as Talbot fringes.
In a Talbot fringe thin-film interferometry method shown in FIG. 1A, a parent diffraction grating pattern 10 is formed on a layer of photoresist 11 using well known techniques, for example by projecting an image of a master grating, or by imprint lithography, or by two-beam interferometric lithography. The photoresist 11 is prevented from being exposed when this grating 10 is formed. Prevention may be accomplished by using imprint lithography, with use of a 2nd wavelength, or, sensitivity may be switched on via diffusion from a substrate underlayer. The photoresist 11 is formed over a dielectric layer 12 (e.g., an oxide, nitride or oxynitride) formed on a Silicon or Silicon—containing material substrate 15. In the structure shown in FIG. 1, an anti-reflective coating 13 such as an absorbing film or a graded index film may be formed as a layer between the dielectric and substrate. Then, using well known techniques, through flood illumination (e.g., with a plane wave) of the parent grating 10, a daughter grating 20 is ultimately patterned as shown in FIG. 1B. If the grating is properly spaced away from the photosensitive film, line/space fringes of doubled frequency are produced. Frequency doubling results from suppression of the zero order for example. In current implementations, the incident illumination is formed by light at wavelengths ranging between about 157 nm and 300 nm. The dimensions of the grating are often of only millimeter scale in experiments but can be 1 cm or larger. The grating pitch is limited by what conventional methods can fabricate, which in the case of projection methods might be in the range of 150 nm.
Extension of the Talbot concept to dimensions of current lithographic interest requires operation outside the scalar Fresnel regime. It has been shown that very small features can be printed in a sensitive layer beneath a parent grating that has been designed to diffract the waves into the +/−1st orders exclusively, thus eliminating all propagation in the normal incidence direction (0th order) and so producing an ideal two-beam interference pattern with half the pitch, as illustrated in FIG. 2. As shown in FIG. 2, elimination of the zero order creates a frequency-doubled fringe pattern 30 that is formed from alternated regions of positive and negative amplitude, as in phase mask lithography. Under ideal circumstances, the minimum achievable spatial frequency, “P” as indicated in FIG. 2, with this technique (in configurations where the standoff is large enough to suppress evanescent waves) is dictated by the index of refraction of the resist medium, according to the equation 1) as follows:
                                          1                          P              min                                =                                    2              ⁢                              n                resist                                      λ                          ,                            1        )            where Pmin is the smallest pitch that can be achieved in the printed line/space pattern. The factor of “2” in equation 1) represents the gain from frequency doubling, while nresist (which may be as large as 1.7 or 1.8, but can potentially be larger), represents an improvement in resolution beyond the effective value of 1.35 which gates current projection lithography. Talbot lithography provides this gain because it does not require propagation of the Talbot fringes through an immersion liquid; instead these frequency-doubled fringes are created at the upper surface of the wafer film stack shown in FIG. 1.
In the scalar Fresnel regime, suppression of the zero order simply means matching the areas of the positive and negative transmission regions of the grating. Restriction of the propagating orders to +/−1 will often improve doubled-frequency contrast in the Fresnel regime. The classical scalar structure with such a diffraction pattern is a grating whose transmission profile is given a sinusoidal form, producing a pure intensity sinusoid (plus DC) at the doubled frequency. However, if the doubled frequency is printed at a depth below the grating that is sufficient to damp out evanescent waves; one may employ variant profiles that include additional non-propagating high frequencies, so long as the propagating spectrum has the desired two-beam form. If nresist is 1.8, the resulting doubled spatial frequency will be considerably finer than can be achieved with current prior-art projection lithography. Note that the grating must be fabricated on the film stack without exposing the imaging resist layer. In the prior art, this has been accomplished by forming the Talbot fringes using a different wavelength from that used to print the grating, so that resists with different spectral sensitivities can be used in the two steps.
One drawback to Talbot lithography is that the printed features are restricted to periodic line/space patterns. Most useful semiconductor structures involve more complex features. While there is a significant subset of semiconductor devices whose design layouts are relatively simple (and moreover such devices may be of particular importance at the ultra-high resolutions where difficulties in device scaling make large and complex circuit layouts more problematic), pure line/space patterns are only of limited utility.
A second difficulty arises in fabricating the parent grating. This must contain structure as fine as half the desired output Talbot spatial frequency, which for a wavelength of 193 nm and a photoresist index of refraction nresist=1.8, must be of the order of 100 nm if one wishes to approach the limit allowed by equation 1. Some of the state-of-the-art techniques described above can provide such fine resolutions, which can then be reduced further through Talbot lithography; however such spatial frequencies for the parent grating fall near the limit of current lithographic technology, and can prove difficult to manufacture. A more fundamental difficulty is that future semiconductor technologies will require printing pitches that are considerably smaller than the wavelength, and under such circumstances electromagnetic effects due to interaction of the incident light with the resist topography produces an undesired increase of the energy diffracted into the 0th order, hence critically degrading the double-frequency Talbot image in this regime. It has been shown that the 0th order tends to increase rapidly as the grating pitch decreases below 180 nm. For this reason a standard grating structure will generally be unable to provide the sub-wavelength frequency-doubled fringe pattern that is needed to achieve a resolution superior to current projection technology.
A further antecedent projection lithographic technology in the prior art include: Thin Film Thin Film Interference Lithography using Surface Plasmons. In this technique, complex electromagnetic effects also arise when light is transmitted through grating arrays (1D or 2D) of pinholes or slits in metallic films, particularly metals whose dielectric constant has a large negative real part. The waves that are excited can be understood as evanescent spatial frequencies, allowing the gratings to be handled with a wave-based numerical methodology.
Recently, a contact lithography process was demonstrated where surface plasmons were launched on a grating array manufactured out of silver, a plasmonic metal, in such a way that they could interfere to produce a standing wave that could be used as an aerial image to expose a thin resist. FIG. 3 shows an example in the form of a 2D FDTD (Finite Difference Time Domain) simulation, where a 60 nm layer of silver 40, for example, has been patterned with 60 nm openings 45, for example, on a 300 nm period. The silver is then exposed with 436 nm wavelength light. This causes surface plasmons to be created on the top and bottom surfaces. On the bottom surface, counter propagating surface plasmons interfere to present a standing wave 48 that can be used to expose the underlying resist.
Thus far only gratings have been proposed for plasmonic lithography, limiting this technique to very specialized applications. And, as with Talbot parent gratings for sub-wavelength spatial frequencies, analysis of the plasmonic gratings is numerically intensive.
Contrast Enhancement Layers
A known method for improving contrast in a projected image of given shape (i.e. composed of a given set of bright and dark features) is to project the image through a bleachable film that is placed just above the resist layer which is used to capture the image. The exposing set of bright and dark features is imprinted into the bleachable film as a transmission profile, so that the film transmits more light where bright features pass through it, and less light where the features are dark. The sidewalls of the transmitted image are also sharpened. Such a bleachable film is known as a Contrast Enhancement Layer (CEL). Variant CEL materials are known that exhibit a thresholded behavior in their bleaching. CELs thus comprise photosensitive films that can sharpen the sidewalls of an exposing pattern after being optically imprinted with an initial pattern. A CEL layer does not change the pitch of the transmitted image (by design the basic pattern of bright and dark features is unchanged with CELs).
Solid Immersion Lithography
It is known in the art that one can avoid the need to propagate an NA>1 image through an immersion medium by using so-called solid immersion lithography. Here the flat outside surface of the final lens element is placed in direct contact with the resist stack on the wafer. (A lens element with such a flat surface is referred to as “plano”.) Though this contact might well be accomplished using a high index liquid, the thickness of the contacting layer is made sufficiently small as to cement the lens to the film stack, and such microscopic thicknesses in the cementing layer ease many of the requirements that a high index coupling medium needs to fulfill in the macroscopic thicknesses required for conventional immersion operation.
A significant problem with the solid immersion approach however, is that large-NA projection systems can only be optically corrected to the diffraction limit over fields that are quite small; typically somewhat smaller than a single chip, and far smaller than a silicon wafer. This means that a relatively small instantaneous image must be scanned across the wafer (in synchrony with a scanned mask) in order to expose a full chip area, and further that this chip-exposure must be stepped out across the wafer for later batch processing of the printed chip array. Unfortunately, a microscopic liquid layer prevents relative motion between the lens and wafer, and it cannot be rapidly applied or released.
Thus, it is the case that lithographic technology is currently constrained by limits on the spatial frequency content of projected images. However, reduction mask projection technology provides a number of advantages, including a well-developed logistical infrastructure for efficiently manufacturing circuit structures of prescribed shape, and the ability to exploit phase tiling to double the spatial frequency limit. Solid immersion lithography is relatively impractical, and can only be expected to extend the spatial frequency limit of projection systems by a small margin since the refractive index of the final lens element that is contacted to the resist stack is currently limited to n=1.56 or below. Non-projection systems have various drawbacks, e.g., Talbot lithography (a/k/a in-situ interference lithography). Talbot lithography is relatively inflexible in the patterns it can produce, and it cannot easily provide high contrast frequency doubling as feature sizes become strongly sub-wavelength, due to EMF enhancement of the zero order at these dimensions.
As known, Plano final surfaces are highly desirable in immersion systems that use a macroscopic coupling fluid, and in solid immersion systems such surfaces are inherently required. From a fundamental point of view, the reason that the refractive index of the final lens element imposes a limit on the resolution of solid immersion lenses is that the power in the final surface needs to be zero (i.e., the element must be plano), in order that the final surface can be optically contacted to the wafer stack. In lenses with a conventional macroscopic coupling space, it is possible to include power in a macroscopically flat final surface by applying a diffractive structure, i.e. by forming a Fresnel lens of concentric rings in the flat surface. However, such a lens element (alternately referred to as a diffractive or holographic element) is not suitable as a final contacted lens surface in a lithographic system that seeks to overcome the limitations imposed by the refractive index of the exit space by placing the final element in close proximity to the wafer. One reason is that a microscopic offset between the macroscopic Fresnel lens and the macroscopic image field implies that the Fresnel lens and image field must have almost exactly the same size. However, the Fresnel lens increases the NA from, for example, 1.35 to 1.8, and this must necessarily demagnify the field size in the same ratio. In a conventional configuration, the holographic element can be spaced away from the image field and given a larger diameter, but this is not possible in a solid immersion system where materials limitations force the high index space between the hologram and image to be quite thin. A related problem is that the aberrations in a Fresnel lens of such high power would be impossible to correct in a telecentric system. A Fresnel lens is given its power by a radial variation in spacing between diffracting fringes, and when the Fresnel lens is placed in contact with the image, its diffracting fringes take the form of a locally varying grating, whose aberrations vary over the field in a complicated way that differs from those of the conventional refractive surfaces in the remainder of the lens that might otherwise be used to correct it.
One could contemplate resolving this problem by using purely holographic imaging, i.e. by creating a customized (information-rich) hologram of the desired wafer pattern, which due to the microscopic standoff would essentially take the form of a moderately defocused waveform of the image. Such a configuration would involve no projection lens, so difficulties in stepping or scanning a contacted wafer would not arise. The hologram would inject its information into the film stack at only a microscopic distance from the imaging layer.
The drawback to such a holographic approach is that the problem of fabricating an ultra-high-resolution image is simply re-posed as that of fabricating the hologram. For this reason, the conventional holograms do not offer the desired path to improved resolution, since the resolution needed to fabricate them is in general no coarser than the resolution attainable in the diffracted patterns that they can form.
It would be highly desirable to provide an improved system and method for forming lithographic images of integrated circuit patterns, and more particularly, what is needed is an improved holographic projection lithographic system for high resolution patterning at sub-resolution dimensions.
It would be highly desirable to provide an improved system and method for forming lithographic images that implements holographic elements for frequency re-doubling.